1. Field of the Invention
The present invention relates to a display device with voltage-controlled active-matrix drive, having a capacitive load, such as in an active-matrix type liquid-crystal display device, and more particularly to a drive circuit for such a display device.
2. Background Art
From the standpoint of extending battery life, there is a serious need to reduce the power consumption in portable information equipment, such as mobile telephones, personal digital assistants (PDAs), and laptop computers. With improvements in processing performance and usable functionality, portable information products, have come to require improved display capabilities, with both high display quality and a larger number of display colors. For this reason, in order to meet the demands for improved display capabilities, active-matrix type liquid-crystal displays (hereinafter referred to as TFT-LCDs) implemented using thin-film transistors (TFTs) are beginning to see use in even such portable information products.
A liquid-crystal panel in a TFT-LCD device (hereinafter referred to as a TFT-LCD panel) has a pair of mutually opposing substrates (a first substrate and a second substrate). These substrates are held fixed with a prescribed distance therebetween (typically several μm) with a liquid-crystal material forming a liquid-crystal layer so as to fill the space between the substrates. At least one of these substrates is transparent, and in the case of making a transmissive-type display, both substrates must be transparent. In a TFT-LCD panel, the first substrate is provided with a plurality of mutually parallel scanning signal lines and a plurality of image signal lines, which are perpendicular to the scanning signal lines. Pixel electrodes are provided at the intersection locations between scanning signal lines and image signal lines, as are pixel TFTs, which serves as switching elements for the purpose of making electrical connection between the pixel electrodes and the corresponding image signal line. The gate terminal of a pixel TFT is connected to a scanning signal line the source terminal of the pixel TFT is connected to an image signal line, and the drain terminal of the pixel TFT is connected to the pixel electrode.
A common electrode, serving as an opposing electrode, is formed over the entire surface of the second substrate, which opposes the first substrate. An appropriate voltage is applied to the common electrode by a common electrode drive circuit, so that a voltage corresponding to potential difference between the pixel electrode and the common electrode is applied across the liquid-crystal layer. Because this applied voltage can be used to control the light transmissivity of the liquid-crystal layer, it is possible to create a desired pixel display by applying an appropriate voltage from the image signal lines.
In order to prevent deterioration of the liquid crystal and to maintain display quality, the above-described TFT-LCD panel is driven by AC drive. Specifically, the TFT-LCD panel is driven so that the polarity of the voltage applied to the liquid crystal is reversed, for example every horizontal scan period. Additionally, in order to reduce the amplitude of the voltage on the image signal line, the potential on the common electrode is changed in response to the above-noted AC drive (by applying what is hereinafter referred to as an AC common electrode signal).
However, even if the use of an AC common electrode signal reduces the amplitude of the voltage on the image signal line, there is still a large variation in potential on the image signal lines when the polarity reverses to achieve AC drive. It is therefore necessary for the image signal line drive circuit to have sufficient capacitive load drive capacity to cause this large a potential change with respect to the capacitive load presented by a TFT-LCD panel. For this reason, the image signal line drive circuit consumes a large amount of power, thereby hindering the achievement of low power consumption in a TFT-LCD display.